This invention relates to the use of a microlens array coupled with an associated detector array to effect color separation within each optical pixel of an area array of detection pixel elements (a focal plane array). This invention relates more particularly to a simpler method for simultaneously detecting radiation intensity in each of several wavelength bands when the focal plane array is illuminated with radiation over a broad spectral range. This invention will be particularly useful for multicolor detector arrays operating in the infrared spectral range.
Traditional sensor systems have relied on filters and beam splitters to direct light in different spectral bands to separate one-color focal plane arrays. Multicolor focal plane arrays therefore provide a large reduction in system complexity in such systems by eliminating complex optics and critical alignments.
Multicolor focal plane arrays have been proposed and some have been demonstrated using a variety of techniques. Perhaps the most straightforward approach utilizes stacked detectors with differing spectral responses, each of which cover most of the optical pixel area. This requires a detector material technology that allows tuning of the spectral response. For example, stacked detector solutions to two-color focal plane arrays are being developed with Mercury Cadmium Telluride and with Quantum Well Infrared Photo-conductor detectors.
While versatile, stacked detector technologies are limited either in response wavelength and/or performance and are therefore not suited for many applications, particularly because of the added materials complexity involved in the fabrication of stacked detectors.
Special patterned filters are often used to make multicolor focal plane arrays. Multicolor charge-coupled device or active pixel sensor focal plane arrays with deposited filters are commonly used in both video and still cameras. However, patterned filters waste photons by rejecting all out-of-band photons and not allowing those to be detected and high performance systems usually need to utilize every available photon for maximum sensitivity. In addition, for long wavelength infrared applications the materials used to fabricate the filter are difficult to deposit and usually require a very thick filter, limiting the transmission and fill factor of the filter as well as the minimum filter size.
Another approach superimposes a linear diffraction grating onto a refractive microlens for each optical pixel and etches an array of the resulting structures, most often on the backside of the detector substrate. The lens focuses the incident light to a spot much smaller than a pixel while the grating simultaneously diffracts the light off-axis at an angle dependent on the wavelength. Sub-pixel sized detector elements on the front side of the substrate at the focus of the lens and are properly spaced to intercept the diffracted light may then detect the light in two or more spectral bands. For example, the structure resulting from a superposition of a linear diffraction grating onto a refractive microlens is referred as a "dispersive microlens" in the disclosure of Gal, U.S. Pat. No. 5,497,269.
These approaches to multicolor lens detector systems are often used but have many drawbacks. They are more complicated because they require multiple optical devices to implement a multicolor detector. The patterned filters are inefficient while the other approaches pose fabrication and materials difficulties and are therefore expensive as well. Furthermore, these methods are difficult to apply over wide spectral bands and to extend into the very long wavelength infrared (VLWIR) spectral region; for example it is difficult to make a linear diffraction grating effective over more than an octave in wavelength.